Patent Application
20220349820
This present disclosure relates to the field of a metal ion detection technology, and specifically to an electrochemical plasmonic fiber-optic sensing apparatus, system and method for detecting metal ions in a solution.
With the rapid development of economy, a large number of wastewater is discharged, causing accumulation of potentially toxic metals, such as heavy metals, in the soil and in the underground water, which in turn results in increasingly severe pollution of these metal ions. For example, some metals, such as lead (Pb), mercury (Hg), manganese (Mn), cobalt (Co), nickel (Ni) copper (Cu), zinc (Zn), or Chromium (Cr), etc. are toxic, even in low concentrations, non-biodegradable, universally distributed, and cause significant harm to human and animal health and the environment. For instance, when the ions of these metals are dissolved in water enter the biosphere and are ingested into an organism, they can be highly detrimental to human health. Therefore, the rapid and accurate detection of the ions of these metals has become an important issue.
At present, the monitoring methods for these metal ions in water are mainly spectroscopy (Karami H et al., 2004), chromatography (Yi R et al., 2018) and electrochemical analysis (Shao Y et al., 2010). Among these, spectroscopy and chromatography are cumbersome, time-consuming and the detection systems are expensive. The anodic stripping voltammetry (ASV) for electrochemical analysis is currently the most widely used method (Hwang J H et al., 2019). However, theses electrochemical methods suffer from unsuppressed electrochemical noise and unavoidable environmental interferences which affect the electrode measurement process, thereby strongly limiting their limit of detection (LOD).
In view of the disadvantages associated with existing metal ions detection approaches, this present disclosure provides a fiber-optic sensing apparatus, system, and method for the detection of metal ion(s) in a solution, such as in a aqueous solution.
In one aspect, the present disclosure provides a sensing apparatus for selectively characterizing at least one metal ion in a solution, which includes a fiber-optic sensor and a controller. The fiber-optic sensor includes an optical fiber and a coating assembly over an outside of the optical fiber. The coating assembly is electrically conductive and active to surface plasmon resonance (SPR), and the fiber-optic sensor is configured, when in contact with the solution, to generate surface plasmon waves at an interface between the coating assembly and the solution upon a compatible input light shedding into and propagating in the optical fiber. The controller is electrically connected with the coating assembly of the fiber-optic sensor, and is configured to provide an adjustable potential thereto such that when the coating assembly of the fiber-optic sensor is in contact with the solution, redox reactions of each of the at least one metal ion occur on an outer surface of the coating assembly, resulting in a detectable change of the surface plasmon waves generated in the fiber-optic sensor, wherein the change of the surface plasmon waves contains information of the each of the at least one metal ion in the solution.
Herein, in the sensing apparatus, the optical fiber can comprise a core and a cladding surrounding the core, and the core is provided with a tilted grating having an inclination angle of more than approximately 2°, and preferably in a range of approximately 6°-22°.
Optionally, the fiber-optic sensor further includes a mirror having a reflective surface facing to a light incident surface of the optical fiber. The mirror is configured to reflect optical signals generated and transmitted in the optical fiber back towards the light incident surface of the optical fiber.
Herein, the coating assembly can include a base film layer, which is configured to be both electrically conductive and active to surface plasmon resonance (SPR). Herein, the base film layer can optionally a metal film layer comprising at least one of gold (Au), silver (Ag), platinum (Pt), copper (Cu) or aluminum (Al), or optionally can comprise a semiconductor material, a metal oxide material, a two-dimensional (2D) material, or an optical metamaterial. The base film layer can have a thickness in a range of approximately 20-70 nm, and preferably in a range of approximately 30-50 nm.
In the sensing apparatus, the coating assembly can optionally further include a conductive protective film layer over an outer surface of the base film layer, which is configured to protect an integrity of the base film layer. The protective film layer can comprise a diamond film layer or a silicon film layer, or can comprise at least one of indium tin oxide (ITO), zinc peroxide (ZnO2), tin oxide (SnO2), or indium oxide (In2O3).
In the sensing apparatus, the coating assembly can optionally further include a transition film layer sandwiched between the optical fiber and the base film layer, which is configured to improve adhesion of the base film layer to the optical fiber. The transition film layer can comprise at least one of titanium (Ti), molybdenum (Mo), or chromium (Cr).
In the sensing apparatus, an outer surface of the coating assembly can optionally modified to have an increased specific surface area. As such, the outer surface of the coating assembly can comprise a plurality of subtractive microstructures and/or a plurality of additive microstructures. If a plurality of subtractive microstructures are included in the modified outer surface, they can include porous microstructures or winkle-like microstructures, or both. If a plurality of additive microstructures are included in the modified outer surface, they can comprise nanoparticle microstructures, nanotube microstructures, or nanofilm microstructures, or any of their combinations.
Examples of the composition of the plurality of additive microstructures can comprise graphite, graphene, carbon nanoparticles, carbon nanotubes, a metal oxide, a two-dimensional material, an optical metamaterial, or any of heir combinations.
In the sensing apparatus, the controller can comprise an electrochemical station, and the fiber-optic sensor serves as a working electrode of the electrochemical station. The electrochemical station can further include a reference electrode and a counter electrode. The metal ions that can be detected by the fiber-optical sensing apparatus includes ions of metals lead (Pb), mercury (Hg), copper (Cu), zinc (Zn), cobalt (Co), ion (Fe), nickel (Ni), arsenic (Ar), chromium (Cr), etc. Preferably, the metal ions include one or both of Pb2+ and Cu2+.
In another aspect, a sensing system is further provided. In addition to the sensing apparatus as described above, the sensing system also includes a light source apparatus and a signal detection apparatus. The light source apparatus is optically coupled to a first end of, and is configured to provide the input light into, the optical fiber in the fiber-optic sensor of the sensing apparatus. The signal detection apparatus is coupled to the sensing apparatus and is configured to receive signals of the surface plasmon waves therefrom so as to derive the information of the each of the at least one metal ion in the solution.
Herein, the optical fiber in the fiber-optic sensor of the sensing apparatus can comprise a core and a cladding surrounding the core, and the core is provided with a tilted grating.
In the sensing system, the light source apparatus can include a light source, a polarizer, and a polarization controller, which are sequentially along an optical pathway into the optical fiber in the fiber-optic sensor, and are arranged such that the input light emitted from the light source becomes a polarized light having a polarization direction substantially parallel to an inscription direction of the tilted grating in the core of the optical fiber.
According to some embodiments, the light source comprises a broadband source (BBS), and the signal detection apparatus comprises an optical spectrum analyzer (OSA). According to some other embodiments, the light source comprises a tunable laser source (TLS), and the signal detection apparatus comprises an optical detector and an analog-to-digital converter. The optical detector is configured to detect, and to convert into analog electrical signals, the signals of the plasmon waves from the sensing apparatus; and the analog-to-digital converter is configured to convert the analog electrical signals into digital electrical signals.
According to some embodiments, the signal detection apparatus is coupled to the first end of the optical fiber. A second end of the optical fiber opposing to the first end is provided with a mirror having a reflective surface facing to, configured to reflect optical signals generated and transmitted in the optical fiber back towards the first end of the optical fiber. The sensing system further comprises an optical fiber circulator, which is optically arranged between the light source apparatus and the sensing apparatus along an input optical pathway and between the sensing apparatus and the signal detection apparatus along an output optical pathway, and is configured to separate the input optical pathway and the output optical pathway to allow the signal detection apparatus to obtain the signals of the surface plasmon waves from the sensing apparatus without being influenced by the input light.
According to some other embodiments, the signal detection apparatus is coupled to a second end of the optical fiber.
In the sensing system, optionally the signal detection apparatus is further configured to receive signals of other optical waves transmitted in the core of the optical fiber (i.e. core mode optical signals), which can be used for their calibration over noise information in the information of the each of the at least one metal ion in the solution. Herein the noise information can include a temperature or light intensity jitter of the light source apparatus
In the sensing system, a measurement range for a metal ion concentration in the solution can be from approximately 10−4 M to approximately 10−10 M, and a limit of detection (LOD) for the concentration of a metal ion in the solution can be lower than 10−10 M.
In yet another aspect, a method for selectively characterizing at least one metal ion in a solution using a sensing apparatus as described above is provided. The method includes the following steps:
(1) arranging the coating assembly of the fiber-optic sensor to be in contact with the solution;
(2) providing the input light into the fiber-optic sensor of the sensing apparatus;
(3) recording an amplitude of the surface plasmon waves transmitted from the fiber-optic sensor when providing, by means of the controller, a first potential to the fiber-optic sensor to allow each of the at least one metal ion to be reduced into a solid metal element corresponding thereto onto the outer surface of the coating assembly, until the amplitude increases to a first plateau;
(4) recording an over-time change of the amplitude of the surface plasmon waves comprising a first sequence of sequentially decreasing plateaus when providing, by means of the controller, a second potential changing over time in a direction reverse to the first potential to the fiber-optic sensor to allow the solid metal element corresponding to each of the at least one metal ion to be oxidized to thereby strip from the coating assembly into the solution, until the amplitude decreases to a last of the first sequence of plateaus; and
(5) analyzing the over-time change of the amplitude of the surface plasmon waves between the first plateau and the last of the first sequence of plateaus to thereby characterize the at least one metal ion in the solution.
Optionally, the first potential can be a substantially constant potential, and the second potential can be configured to change at a substantially constant rate over time.
In the method, step (5) can include the following sub-steps:
taking a derivative calculation at each point of a curve corresponding to the over-time change of the amplitude of the surface plasmon waves between the first plateau and the last of the first sequence of plateaus to thereby obtain a derivative-over-time curve; and
determining an identity of each of the at least one metal ion by identifying a characteristic trough on the derivative-over-time curve, wherein the trough corresponds to a locally fastest amplitude change thereon.
In the method, step (5) can include the following sub-steps:
calculating an amplitude change of the surface plasmon waves between each pair of two neighboring plateaus in a second sequence of plateaus sequentially comprising the first plateau and the first sequence of plateaus; and
determining a concentration of each of the at least one metal ion in the solution by plotting the calculated amplitude change corresponding to each pair of two neighboring plateaus in the second sequence of plateaus against a pre-determined standard curve, wherein the pre-determined standard curve is obtained in advance by plotting a set of sample solutions, each with a known yet different concentration of the each of the at least one metal ion.
Herein the standard curve can be a linear curve obtained by plotting an amplitude change of the surface plasmon waves relative to Log M for each sample solution, where M is a concentration of the each of the at least one metal ion in the each sample solution.
Prior to step (5), the method can further include a step of recording signals of other optical waves transmitted in the core of the optical fiber in the fiber-optic sensor, and step (5) can include:
performing calibration to the over-time change of the amplitude of the surface plasmon waves;
analyzing the calibrated over-time change of the amplitude of the surface plasmon waves between the first plateau and the last of the first sequence of plateaus to characterize the at least one metal ion in the solution.
The following are noted throughout the disclosure. The terms “device”, “apparatus”, and alike are considered to be exchangeable. the terms “amplitude”, “intensity”, and alike are considered to be exchangeable where they are used for description of a strength of an optical signal. The term “solution” is considered as a working system where the metal ions can exist in the form of ions. The term “at least one metal ion” is referred to as at least one type of ions of a metal, including ions of different metals (e.g. Pb and Cu), and also including ions with different electron configurations for a same metal (e.g. Fe2+ and Fe3+).
In a first aspect, this present disclosure provides a fiber-optic sensor configured to detect metal ions contained in a solution. The fiber-optic sensor includes an optical fiber and a coating assembly that coats an outside of the optical fiber. The coating assembly is configured to be active to surface plasmon resonance (SPR), and the fiber-optic sensor is configured, when in contact with the solution, to generate surface plasmon waves at an interface between the coating assembly and the solution upon a compatible input light shedding into and propagating in the optical fiber.
The coating assembly is further configured to be electrically conductive to thereby allow the fiber-optic sensor to be used as a working electrode when characterizing the metal ions in the solution, and an outer surface of the coating assembly substantially provides a surface where redox reactions (i.e. reduction reaction and oxidation reaction) of the metal ions occur, causing a detectable change of the surface plasmon waves generated in the fiber-optic sensor. Detection of the change of the surface plasmon waves can derive information of the metal ions in the solution, such as the determination of the identities and quantifications of the metal ions contained in the solution. In the fiber-optic sensor disclosed herein, the optical fiber can have a tilted grating in the core thereof, but can also be of other types. There are no limitations herein.
Specifically regarding the redox reactions of the metal ions H1 in the solution, in a reduction step (or deposition step) of the detection method using the fiber-optic sensor 50, the metal ions H1 in the solution S are substantially reduced to becoming a corresponding solid metal element (or a metal solid substance) H0 that is deposited onto the outer surface of the coating assembly 30, whereas in an oxidation step (or stripping step) of the detection method (as provided below in more detail), the solid metal element H0 that attaches onto the outer surface of the coating assembly 30 of the optical fiber can be oxidized to becoming the metal ions H1 again that are stripped from the coating assembly 30 of the fiber-optic sensor 50 and dissolved into the solution S.
In this embodiment of the fiber-optic sensor, the core 10 of the optical fiber is provided with a tilted grating 12, i.e. a grating having an internal tilt angle θ (defined as an angle of each plane of the grating relative to a plane that is substantially perpendicular to the axis of the core 10). Upon an input light 1 entering from a first side surface A into the optical fiber and transmitting along the core 10, the tilted grating 12 can reflect and/or refract the input light into the cladding 20 of the optical fiber (the light such reflected or refracted is shown as 2 in
More specifically, when the metal solid substance H0 on the outer surface of the coating assembly 30 interacts with the plasmon resonance wave 3, the amplitude of the absorption envelope in the cladding mode changes correspondingly, and the direction of the amplitude change is correlated to the deposition and dissolution process of metal solid substance H0 on the surface of the coating assembly 30. The maximum slope of the amplitude change corresponds to the peak value of metal dissolution/stripping, and the amplitude change value is related to the concentration of the metal ions H1 in the solution S. In other words, the state of change of the metal ion ions H1 on the surface of fiber-optic sensor 50 can be modulated by the wavelength amplitude of the absorption envelope of the plasma resonance wave. Specifically, by obtaining a derivative of the amplitude change of the surface plasmon resonance, it can clearly identify and detect the stripping peak potential of the metal ions H1 to thereby realize a specific recognition of the metal ions H1. As such, the detection of the metal ions H1 can be converted from a current-based detection to an optical-electrochemical signal-based detection, so the detection system disclosed in this embodiment can simultaneously obtain optical and electrochemical quantities and can be analyzed to obtain the internal relationship therebetween.
In addition, the fiber-optic sensor 50 can also reflect optical waves at certain wavelengths in the core 10 of the optical fiber (i.e. core-mode optical waves, not shown in the above drawings) which, if detected, can be used as an inherent reference when doing the analysis of the surface plasmon waves 3 to thereby remove the unwanted influence, or interference, due to fluctuations from certain factors, such as those from the environment (e.g. temperature) or those from the sensing system (e.g. light source level). As such, the fiber-optic sensor 50 disclosed herein can have a feature of being capable of self-calibration or self-correction.
The fiber-optic sensor 50 also has a second side surface B opposing to the first side surface A, and could be a light emitting surface (e.g. for a transmission-mode optical fiber), or could be a light reflecting surface (e.g. for a reflection-mode optical fiber). In the latter case, a mirror can be arranged on the second side surface, and will be described below in more detail. The cross-sectional view of the fiber-optic sensor 50 as shown in
As further illustrated in
Optionally, according to a second configuration (II) illustrated also in
Optionally, according to a third configuration (III) illustrated also in
For this purpose, according to different embodiments, the outer surface of the coating assembly can be configured to comprise one or both of a plurality of subtractive microstructures and a plurality of additive microstructures. Herein and throughout the disclosure, a microstructure is defined as a fine structure having a scale of nanometers or micrometers.
Specifically, as illustrated in the embodiment (IV) in
As further illustrated in the embodiments (V and VI) shown in
Herein, the input light 1 as referred to above and illustrated in
Furthermore, in any of the embodiments of the fiber-optic sensor as described above, the optical fiber can have components, compositions, dimensions, and/or configurations of the optical fibers mentioned in any of the embodiments that follow, such as those use for telecommunications-grade optical fiber (e.g. Corning SMF-28), but can also have other parameters.
In the three embodiments of the coating assembly 30 as described above, the base film layer 31 can have a thickness in a range of 20-70 nm, for example, in a range between 30-50 nm. The tilted grating 12 in the optical fiber of the fiber-optic sensor 50 can be obtained by means of an excimer laser and a phase mask, or by a double beam interference; and the tilted grating 12 can have an inclination angle of more than approximately 2°, and preferably between 6°-22°. The metal ions that can be detected by the fiber-optic sensor disclosed herein can include ions of both heavy metal and transition metals. Non-limiting examples of the metal whose ions can be detected by the fiber-optic sensor include including lead (Pb), mercury (Hg), copper (Cu), zinc (Zn), cobalt (Co), ion (Fe), nickel (Ni), arsenic (Ar), chromium (Cr), etc. It is also possible to differentiate between the different type of ions of a same metal, such as Fe2+/Fe3+, Co2+/Co3+, Cu+/Cu2+, etc. Other metal ions are also possible.
In a second aspect, a fiber-optic metal ion detection apparatus (or referred to as “detection apparatus”, “sensing apparatus” throughout the disclosure) comprising any of the embodiments of the fiber-optic sensor as described above in the first aspect is further provided.
In the reflection mode shown in
In a third aspect, a sensing system comprising any of the embodiments of the fiber-optic metal ion detection apparatus as described above in the second aspect is provided.
In addition to the fiber-optic metal ion detection apparatus, the sensing system further includes a light source apparatus and a signal detection apparatus, which are both optically and communicatively coupled with the fiber-optic metal ion detection apparatus, and are configured respectively to provide an input light into the sensing apparatus, and to receive signals of the surface plasmon waves from the fiber-optic sensing device, so as to derive the information of the metal ions in the solution for the identification and quantification thereof. More specifically, the light source apparatus is optically coupled to a first end of, and configured to provide an input light into, the fiber-optic metal ion detection apparatus so as to allow the electromagnetic radiation to propagate in the optical fiber of the fiber-optic sensor of the fiber-optic metal ion detection apparatus; and the signal detection apparatus is coupled to the fiber-optic metal ion detection apparatus and configured to receive the signals of the surface plasmon waves therefrom.
Depending on the two different working modes (i.e. the reflection mode and the transmission mode) of the fiber-optic sensor in the fiber-optic metal ion detection apparatus, the sensing system has different configurations.
In both of
In any one embodiment of the fiber-optic sensing system 100 described above, the light source apparatus 300 can include a light source, a polarizer, and a polarization controller (PC). Herein the light source can be a broadband source (BBS) or a tunable laser source (TLS). Light emitted from the light source can be converted into a polarized light having a polarization direction substantially parallel to an inscription direction of the tilted grating after the emitted light transmits through the polarizer and the polarization controller.
According to some embodiments of the fiber-optic sensing system 1000 as illustrated in
According to another embodiments of the sensing system 1000 as illustrated in
The input light is further converted, via the polarizer 320′ and the polarization controller (PC) 330′, into a polarized light with aforementioned polarization direction before it enters into the optical fiber of the sensing apparatus so as to excite surface plasmon waves on the surface of the sensing apparatus 100. The optical detector 210′ is configured to detect, and to convert into analog electrical signals, the signals of the plasmon waves from the sensing apparatus 100. The analog-to-digital converter 220′ is further configured to convert the analog electrical signals into digital electrical signals, based on which an interrogation can be performed over a quantification of intensity variations to thereby derive the information of the metal ion(s) in the solution.
According to some embodiments of the sensing system, the signal detection apparatus is further configured to receive signals of other optical waves refracted by the grating and transmitted in the core of the optical fiber in the fiber-optic sensor of the sensing apparatus (i.e. core mode) for the calibration or correction over noise information (temperature, light source jittering, etc.) in the information of the each of the at least one metal ion in the solution.
Using the sensing system as disclosed herein, a measurement range for the concentrations of the at least one metal ion in the solution can be around 10−4-10−10 M, and a limit of detection (LOD) of <10−10 M can be achieved. In addition, the metal ions that can be characterized using the sensing system disclosed herein can comprise ions of one or more of lead (Pb), mercury (Hg), copper (Cu), zinc (Zn), cobalt (Co), ion (Fe), nickel (Ni), arsenic (Ar), or chromium (Cr).
In a fourth aspect, a metal ion detection method utilizing the fiber-optic sensing apparatus as described above in the second aspect is further provided.
S100: Setting up a fiber-optic sensing apparatus, and arranging the coating assembly of the fiber-optic sensor to be in contact with the solution;
S200: Providing an input light into the fiber-optic sensor;
S300: Providing a reducing potential to the fiber-optic sensor and recording an amplitude of the surface plasmon waves transmitted from the fiber-optic sensor, until the amplitude increases to a first plateau;
S400: Providing an oxidizing potential that changes over time to the fiber-optic sensor, and recording an over-time change of the amplitude of the surface plasmon waves until the amplitude decreases to a last plateau; and
S500: Analyzing the over-time change of the amplitude of the surface plasmon waves between the first plateau and the last plateau to characterize the at least one metal ion in the solution.
In the method as described above, each of the reducing potential (in step S300) and the oxidizing potential (in step S400) is provided to the fiber-optic sensor (more specifically, the coating assembly) by means of the controller in the fiber-optic sensing apparatus. The reducing potential is substantially a first potential which, upon application to the fiber-optic sensor by the controller, allows each of the at least one metal ion in the solution to be reduced into a corresponding solid metal element that deposits onto the outer surface of the coating assembly. The oxidizing potential is substantially a second potential which, upon application to the fiber-optic sensor by the controller, allows the solid metal element corresponding to each of the at least one metal ion to be oxidized to thereby become its ion form to thereby strip from the coating assembly into the solution. The oxidizing potential is configured to change over time in a direction reverse to the reducing potential. Optionally, the reducing potential is configured to be a substantially constant potential, and the oxidizing potential is configured to change at a substantially constant rate over time.
Herein, characterization of the at least one metal ion in the solution can include identification, i.e. determination of the identity, of each metal ion in the solution, and can also include quantification, i.e. determination of the concentration, of each metal ion in the solution.
In the method disclosed herein, the determination of the identity of one particular metal ion relies on the identification of a locally fastest change of the amplitude of the surface plasmon waves. It can be realized by taking a derivative at each point of a curve corresponding to the over-time change of the amplitude of the surface plasmon waves between the first plateau and the last plateau to thereby obtain a derivative-over-time curve. The identity of each metal ion can be determined by identifying a characteristic trough (or peak) corresponding thereto on the derivative-over-time curve. Each trough (or peak) substantially corresponds to a locally fastest amplitude change on the derivative-over-time curve, with a characteristic position.
If there is only one metal ion to be investigated, there is only one trough, whose position on the derivative-over-time curve can be used to determine the identity of the metal ion. If there are two or more metal ions in the solution, an equal number of troughs can show up on the derivative-over-time curve, each with its characteristic position telling the identity of each metal ion in the solution.
In the method disclosed herein, the determination of the concentration of one particular metal ion relies on the calculation of the amplitude change of the surface plasmon waves on the amplitude-over-time curve, which is correlated to the concentration of the metal ion.
Specifically, using the method as described above, if there is only one metal ion to be investigated, only two plateaus (i.e. the first plateau, and the last plateau) in the amplitude-over-time curve are observed, and such amplitude decrease (i.e. the difference between the first plateau and the last plateau) can be plotted against a pre-determined standard curve to thereby obtain the concentration of the metal ion. Herein the pre-determined standard curve is obtained in advance by plotting a set of sample solutions, each with a known yet different concentration of the same metal ion. Optionally, the standard curve can be a linear curve, obtained by plotting an amplitude change of the surface plasmon waves relative to Log M for each sample solution, where M is a concentration of the same metal ion in each sample solution.
Using the method as described above, if there are N (N>1) metal ions to be investigated, one the amplitude-over-time curve, there will be N−1 plateaus between the first plateau and the last plateau, together forming a sequence of plateaus [P0, P1, P2, . . . , PN] in a sequentially decreasing manner, where P0 represents the first plateau and PN the last plateau. In order to calculate the concentration for each of the N metal ions, a sequence of N amplitude changes are obtained, each calculated by taking an amplitude difference between every two neighboring plateaus in the sequence of plateaus, which can be expressed as [P1-P0, P2-P1, . . . , and PN-PN-1]. In a manner similar to the single metal ion concentration calculation, each amplitude change is plotted against a pre-determined standard curve specific to that particular metal ion to thereby obtain the concentration thereof.
In the method as described above, the core-mode optical signals (i.e. the optical waves generated and transmitted in the core of the optical fiber in the fiber-optic sensor) can be optionally utilized for the calibration or correction of noise caused by the environment or the system (such as the temperature or the light source jittering, etc.).
Because the fiber-optic sensor, sensing apparatus, system and method as provided herein relies on an optical fiber sensor, they has advantages such as anti-electromagnetic interference, high sensitivity, low loss, long life, light weight and low cost, etc.
In the following, one specific embodiment of the sensing system is provided as an illustrating example. As illustrated in
A light emitted by the light source 310 sequentially passes through the polarizer 320, the polarization controller 33030 and the optical fiber circulator 40000, and then sheds into the fiber-optic sensor 50 whose core is engraved with the tilted optical fiber grating. A cladding mode generated in the optical fiber is coupled to the metal film to thereby stimulate a surface plasmon resonance (SPR) of the metal film. The fiber-optic sensor 50 evanesces the light containing plasma resonance waves to the environment outside the metal film, which interact with the heavy metal elements (i.e. the solid heavy metal Pb element) attached onto the surface of the metal film to cause energy loss and in turn to change the amplitude of the wavelength of the resonance center. This above phenomenon can be detected in the optical spectrometer 210.
When the heavy metal element on the surface of the metal film interacts with the plasmon resonance wave, the amplitude of the absorption envelope changes correspondingly, and the direction of the amplitude change is related to the deposition and dissolution process of heavy metal element on the surface of metal film. The maximum slope of the amplitude change corresponds to the peak value of heavy metal dissolution, and the amplitude change value is related to the concentration of the heavy metal ion Pb2+ in the solution. In other words, the state of change of the heavy metal ion Pb2+ on the surface of fiber-optic sensor 50 can be modulated by the wavelength amplitude of the absorption envelope of the plasma resonance wave. Specifically, by obtaining a derivative of the amplitude change of the surface plasmon resonance, it can clearly identify and detect the stripping peak potential of the heavy metal ion Pb2+ to thereby realize a specific recognition of the heavy metal ion Pb2+. As such, the detection of the heavy metal ion Pb2+ can be converted from a current-based detection to an optical-electrochemical signal based detection, so the detection system disclosed in this embodiment can simultaneously obtain optical and electrochemical quantities and can be analyzed to obtain the internal relationship therebetween. The sensing system is configured to measure the change of the amplitude of the waves of the SPR to thereby obtain information about the concentration of the heavy metal ion by calculating a maximum value of a rate of the change of the amplitude of the waves of the SPR to determine a peak electrical potential of a stripping process of the heavy metal ion to thereby determine a type of the heavy metal ion.
In this specific embodiment of the sensing system, the light source 310 comprises a broadband source (BBS) having an output spectrum ranging from 1250-1650 nm, and the output spectrum operably matches an envelope range of a transmission spectrum of the tilted grating in the optical fiber of the fiber-optic sensor 50. The tilted grating in the optical fiber of the fiber-optic sensor 50 is obtained by means of an excimer laser and a phase mask, and has an inclination angle of approximately 18 degrees, and has an axial length of 10-20 mm.
Furthermore, in this embodiment of the system, the metal film on the fiber-optic sensor 50 in optical fiber sensing probe is fabricated by magnetron sputtering, and the metal film has a thickness of 30-50 nm, which ensures the best excitation efficiency of plasma. In the process of coating the nanometer-thick metal film, the target is fixed, and the optical fiber rotates at a uniform speed along its own axis to ensure the uniformity of thickness of the metal film. The metal film comprises a gold film, which ensures that it can not only effectively excite the plasma resonance wave, but also have good conductivity. The gold film coated on the outer surface of the cladding of the optical fiber is also very stable. The heavy metal element can attach onto the surface of the gold film well, or can strip from the surface of the gold film. The surface of the gold film can also be modified with nanoparticles or nanofilms, such as graphene, carbon nanotubes, and other two-dimensional materials, so as to increase the specific surface area of the fiber-optic sensor 50, to elevate the ion enrichment capability, and to improve the conductivity.
This embodiment of the disclosure further provides an electrochemical plasmonic fiber-optic sensing method for detecting heavy metal ions in a solution. The method is substantially based on the detection system as described above, and comprises the following steps:
S1: The solution was prepared with different concentration gradients according to the standards, and the after-cleaning fiber-optic sensor 50, the reference electrode 60 and the counter electrode 70 were submerged into the prepared solution containing the heavy metal ion to be tested.
S2: The light source 310, the polarizer 320, the polarization controller 330, and the optical fiber circulator 400 were sequentially and operably connected to thereby set up an optical path, which is configured such that a light emitted from the light source 310 is converted into a polarized light after passing through the polarizer 320, a polarization direction of the polarized light is then adjusted by the polarization controller 330 to be substantially same as a writing direction of the tilted grating in the fiber-optic sensor 50, and lights in the optical path were ensured, by observing changes of an output spectrum of the optical spectrometer 210, to be in a polarized state capable of exciting surface plasma resonance (SPR) on a metal film coated on the optical fiber of the fiber-optic sensor 50.
In this step, the polarized light is a polarized light that is parallel to the writing direction of the tilted optical fiber grating, which was determined by the peak amplitude of the plasma resonance wave excited by the metal film surface on the outer surface of the optical fiber cladding. That is, the peak amplitude of the surface plasma resonance wave is maximum when the polarized light is parallel to the writing direction of the tilted optical fiber grating.
S3: The electrochemical workstation 8 was operably connected to a computer to thereby set up an electrical circuit, and relevant parameters were set up through a software running on the computer. The indoor temperature was controlled to be a normal and constant temperature, and the external environment was also maintained to be constant so that the detection process is not disturbed.
S4: The heavy metal ion detection apparatus 100 was arranged to stand still under a natural condition, and then the optical-based and the electrochemical-based approaches were simultaneously applied to detect the heavy metal ion Pb2+ in the heavy metal ion-containing solution to be tested. The specific operations are as follows:
A constant electric potential of −1.40 V was first applied, by means of the electrochemical workstation 8, to each of the three electrodes (i.e. the working electrode 5, the reference electrode 60, and the counter electrode 70) of the heavy metal ion detection device 10, so as to allow the heavy metal ion (Pb2+) 15 in the solution to, under this voltage excitation, be reduced to the solid substance 14 deposited on the working electrode (i.e. the fiber-optic sensor 50). This process lasted for 230 seconds.
A reverse voltage was then applied to the three electrodes of the heavy metal ion detection apparatus 100 to oxidize the heavy metal Pb solid substance 14 deposited on the working electrode 5, and during the dissolution (or stripping) process, the heavy metal solid substance Pb deposited on the surface of the working electrode are oxidized into heavy metal ion Pb2+ that dissolves back into the solution. An oxidation current is generated in this process, and the highest peak in the voltammetry curve is the stripping peak of the heavy metal ion Pb2+, and correspondingly the change rate of the plasma resonance responsive curve tends to be the largest. The electrochemical workstation 8 and the optical spectrometer 210 were used to record the relevant data of the above process, and a corresponding stripping potential voltammetric curve was drawn, which was used for reference and calibration of the optical output signals of the optical fiber plasma resonance wave.
Specific experiments using the above metal ion detection system are provided below with more details.
Method and System
The optical system for detecting heavy mental ions is an all-fiber-coupled EC-SPR fiber-optical sensing system, as shown in
The detection of heavy metal ions is based on differential pulse anodic stripping voltammetry (DPASV). In DPASV the analyte of interest is first electroplated onto the working electrode before being removed or stripped by applying an oxidizing potential (as a series of voltage pulses with increasing amplitude). In the present case, Pb2+ ions were first deposited onto the surface of the optical fiber electrode at a potential of −1.40 V. Then a reverse voltage was applied to oxidize the metal on the electrode. During the oxidation process, an oxidation current is generated. Since both the deposition and stripping processes for Pb2+ ions occur on the surface of the working electrode (the gold coated optical fiber sensor probe), they accordingly modify the excited surface plasmon resonances. By monitoring the optical transmission of the optical fiber, real-time and continuous monitoring of heavy metal ion concentration with high sensitivity was achieved.
DPASV keeping the parameters of the electrochemical workstation constant was used to ensure the consistency of the optical fiber sensor when detecting concentrations of different solutions. The optical fiber sensor was thoroughly cleaned before each change of solution. For each measurement, the deposition potential was kept at −1.40 V for a duration of 230 s (the optimum deposition time determined from analysis of multiple experimental runs). At the end of the deposition process, the applied potential on the working electrode was immediately switched to −1.30 V and then swept to +0.50 V at a constant rate. The −1.30 to +0.50 potential is approximately centered around the −0.40 V stripping potential for Pb2+ ions (Gomaa H et al., 2018). After cleaning the sensor, the next round of testing can be performed. When the electrochemical workstation is operating in DPASV mode, both the electrochemical and spectroscopic optical signals can be simultaneously recorded.
Results and Discussion
During the above-mentioned stripping process, the reflection spectrum from the optical fiber sensor was continuously recorded. In
The response sensitivity of the EC-SPR optical fiber sensor proposed herein was studied by carrying out two sets of measurements for different concentration of Pb2+ solutions. The first set of tests was for the Pb2+ concentration range from 10−5 to 10−4 M (small dynamic range, high Pb2+ concentration) and the second set of tests covered the Pb2+ concentration range from 10−10 to 10−5 M (large dynamic range, low concentration).
In order to demonstrate the selectivity of the sensor, control experiments were conducted by measuring a mixed solution of heavy metals Pb2+ and Cu2+ with the same concentration of 10−5 M. In the first step, samples were measured with only Cu2+ ions, as shown in
Conclusions: An electrochemical-surface plasmon resonance optical fiber sensor for real-time and high-sensitivity monitoring of heavy metal ions is proposed and experimentally demonstrated. The gold coated optical fiber acts both as a working electrode and highly sensitive SPR sensor. A LOD down to 10−10 M and a linear response over a large range of 10−10˜10−5 M have been achieved. Stable and reproducible correlation between the real-time ion deposition-stripping cycles and the optical transmission of the optical fiber sensor has been proved over repeated tests. This sensor also shows good selectivity potential ability for specific detection together with temperature self-calibration ability. The proposed EC-SPR fiber-optic sensor has the advantages of compact size, flexible shape and remote operation capability, thereby opening the way for other opportunities for electrochemical monitoring in various hard-to-reach spaces and remote environments.
| Number | Date | Country | Kind |
|---|---|---|---|
| 201910437558.9 | May 2019 | CN | national |
The present application is a U.S. national stage application of International Patent Application No: PCT/CN2020/092011 filed on May 25, 2020, which claims priority to Chinese Patent Application No. 201910437558.9 filed on May 24, 2019. The disclosure of these two patent applications is hereby incorporated by reference in their entirety.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/CN2020/092011 | 5/25/2020 | WO |
